Among technologies for removing nitrates nitrogen intended for the purification of water, heterotrophic denitrification with the use of methanol or an organic carbon source in sludge as a hydrogen donor has been known. This a process is influenced by decomposing systems other than the target microorganisms and its denitrifying efficiency per substrate is low; however, it is capable of removing nitrates nitrogen relatively simply and is put to practical use in those treating facilities which are strictly controlled not to discharge methanol or sludge to outside after its use in denitrification. That is, the denitrifying operation according to this process cannot be completed by merely passing water to be treated through a single denitrifying tank and requires many treating steps and tanks and facilities.
In consequence, it has been difficult to apply this process to effluent of underdrains in farms; the effluent in question contains nitrogenous components mostly consisting of inorganic nitrates nitrogen and the concentration of nitrates nitrogen has become an issue in recent years. Moreover, there is the possibility of newly generating water pollution unless a denitrifying substrate is injected precisely in an amount conforming to the flow rate of the effluent under treatment and to the amount of nitrates nitrogen therein contained and the operation of heterotrophic denitrification requires equipment and full-time careful control such as installed and practiced in a sewage treating plant.
In contrast, autotrophic denitrification with the use of sulfur or a sulfur compound is limited to denitrification by sulfur-oxidizing bacteria; hence, it is effected at a high denitrifying efficiency per substrate and produces sulfate ions which are not limiting factors of water quality. Thus, the process offers an advantage that, where the content of sulfate ions is 1% or less, the treated water can be discharged as it is after control of its pH by a calcium compound. An example of sulfur-oxidizing bacteria is Thiobacillus denitrificans and a reaction represented by the following equation is known.1.114S+NO3−+0.669H2O+0.337CO2+0.0842HCO3−+0.0842NH4→0.0842C5H7NO2+0.5N2+1.114SO42−+1.228H+A number of processes for autotrophic denitrification with the use of sulfur or a sulfur compound have been proposed, for example, in the following patent literature; JP (Japan Patent) 62-56798 B (1987), JP 63-45274 B (1988), JP 60-3876 B (1985), JP 01-31958 B (1989), JP 04-9119 B (1992), JP 04-74598 A(1992), JP 04-151000 A (1992), JP 04-197498 A(1992), and JP 06-182393 A (1992).
Of the aforementioned processes, those described in JP 62-56798 B (1987) and JP 63-45274 B (1988) have been developed for treating effluent containing a variety of nitrogen compounds and sulfur compounds and respectively require a pretreatment at pH 3 or less or a step for cultivating activated sludge seeded with a group of sulfur-oxidizing baceria as dominant. In consequence, they cannot be said to be efficient in case the principal target of denitrification is nitrates nitrogen.
The processes described in JP 60-3876 B (1985) and JP 01-31958 B (1989), although not limited to denitrification aimed at nitrates nitrogen, respectively carry out denitrification by sulfur-oxidizing bacteria in the denitrification step following the nitrification step. However, they cannot carry out autotrophic denitrification efficiently because a sulfur component needs to be added in an amount conforming to that of nitrates nitrogen in the object to be treated or minute bubbles of nitrogen gas produced by denitrification cannot discharge by themselves and their discharge separately requires an aeration tank.
The process described in JP 04-9119 B (1992) relates to simultaneous removal of nitrogen and phosphoric acid from effluent with the use of marble composed of calcium carbonate and sulfur particles. However, the particles of marble and sulfur are not used in the same step and the process is fundamentally an aerobic-anaerobic activated sludge treatment. Therefore, unlike autotrophic denitrification with sulfur alone as denitrifying substrate, the process requires control of sludge and is not efficient for direct denitrification of nitrates nitrogen.
The process described in JP 04-74698 A (1992) is fundamentally an anaerobic-aerobic activated sludge treatment, but it performs more stably in removal of nitrates nitrogen than the aforementioned processes because of the introduction of sodium hydrogen carbonate or calcium carbonate as carbon source. The process, however, is not efficient for direct denitrification of nitrates nitrogen since it is low in denitrifying efficiency on account of pyrites being used as sulfur source and, besides, it remains in the category of activated sludge processes requiring a number of treating tanks.
The process described in JP 04-151000 A (1992) relates to autotrophic denitrification by sulfur-oxidizing bacteria with sodium hydrogen carbonate or calcium carbonate supplied as carbon source and a thiosulfate salt used as sulfur source and electron donor. The thiosulfate salt must be injected in an amount conforming to that of nitrates nitrogen in the object to be treated and, like the aforementioned, the process is not efficient for direct denitrification of nitrates nitrogen.
The process disclosed in JP 04-197498 A (1992) relates to autotrophic denitrification by sulfur-oxidizing bacteria as a pretreatment in purification of water. In this case as well, the efficiency drops markedly unless sodium sulfite is added in an amount conforming to that of the object of denitrification contained in raw water and the process cannot be adapted easily to direct denitrification of nitrates nitrogen.
Any one of the processes described thus far does not aim at direct denitrification of nitrates nitrogen as its object and most of them are no better than activated sludge processes and inefficient from the standpoint of denitrification. On the contrary, the process described in JP 06-182393 A(1994) can remove nitrate nitrogen efficiently by sulfur-oxidizing bacteria.
This process, however, uses sulfur powder that is highly reactive toward sulfur-oxidizing bacteria and effects denitrification by passing raw water containing nitrates nitrogen and the like through a fluidized bed reactor vessel in which a layer filled with sulfur powder is formed. Motive power is necessary to release nitrogen gas adhering to and between the sulfur particles in the course of denitrification and, unless the gas is released, the sulfur particles exhibiting good denitrifying efficiency become wrapped in bubbles and cannot participate in denitrification any longer. The same is true for the bubbles between particles. Other difficulties are a necessity for separate correction of the acidity of sulfuric acid being formed and a necessity for forced passage of raw water at all times to prevent the occurrence of high acidity which would stop the denitrification. Thus, general application of this process, for example, to the treatment of effluent of underdrains would be difficult to realize because it would incur considerable capital and running costs.
In addition, the aforementioned processes do not provide any concrete measures for supply of carbon source which is just as important as sulfur source and for control of the pH, namely, keeping the pH in the vicinity of 7, in order to maintain the activity of sulfur-oxidizing bacteria and effect denitrification with certainty.
Recent knowledge obtained in a learned society for water treatment suggests the importance of taking the following actions in order to effect denitrification smoothly by sulfur-oxidizing bacteria.
That is, it is essential to accelerate the multiplication of sulfur-oxidizing bacteria in order to sustain the denitrification reaction by the bacteria in question and, to accomplish this, it is necessary to supply with certainty carbon source that is required for the synthesis of bacteria in addition to sulfur that is a source of nutrient and to control the pH generally at 7 or higher to maintain the microbial activity. Reference should be made to (4-28) Removal of Nitrate Nitrogen from Actual Sewage by Denitrification with Sulfur, the 49th National Meeting for Presentation of Researches on Water Supply, May, 1998, Abstracts of Papers, pp. 238-239 (4. Section of Water Purification); Japan Water Supply Society. However, it is not always necessary to keep the pH at 7 or more as will be described below.
As illustrated above, the most efficient way for safe removal of nitrates nitrogen is autotrophic denitrification by sulfur-oxidizing bacteria, but it has been difficult to produce surely and easily the effect of this process.
Accordingly, an object of this invention is to provide a denitrifying composition for microbially removing nitrates nitrogen which does not require supply of carbon source that is essential to microorganisms, exhibits a good balance in the system before and after the reaction, reduces the influences of pH and chemical substances to the water area, and performs stably.
Another object of this invention is to provide a denitrifying material which can be brought into contact with effluent even during a dearth of water and can maintain a high denitrifying efficiency at all times.
A further object of this invention is to provide a process for producing the aforementioned denitrifying composition in a short time at low cost.